PROJECT SUMMARY/ABSTRACT Recent calls from the NIMH Director and NIMH-sponsored National Action Alliance for Suicide Prevention emphasize a critical need for the identification of biological predictors of suicidal behavior and improved understanding of short-term fluctuations in acute risk. In response, the proposed prospective study explores the role of the menstrual cycle in acute risk for suicidal ideation and behavior in adolescents and the mediating role of social processes. Robust evidence demonstrates perimenstrual (around menses) increases in psychosocial suicide risk factors (e.g. anger, felt rejection) in susceptible women. Prior research has also documented a cross-sectional association between the perimenstrual phase and increased suicidal behavior, but no research has examined suicidal ideation, planning, or behavior longitudinally across the cycle. Furthermore, no research has longitudinally examined menstrual cycle changes in suicidal ideation and behavior or suicide risk-related social processes among adolescent girls, despite exponential increases in prevalence rates of suicide and depression around menarche. The proposed study will examine associations between menstrual cycle phase and suicidal 1) ideation, 2) planning, and 3) intent in adolescent females. Moreover, the proposed work will examine daily disruptions in social processes (including perceived rejection/loneliness and anger/interpersonal conflict) as a mediator of the association between menstrual cycle phase and within-person change in suicidal thoughts and behavior. A total of 40 post-menarcheal adolescent women with current or recent suicidal ideation (but minimal risk for suicide attempt) will complete daily assessments using an experience sampling protocol. Daily surveys will include an assessment of suicide risk-related social processes (i.e., related to RDoC Loss and Social Process constructs) and acute suicidal ideation and behavior. Participants also will provide information on menstrual bleeding and complete simple at-home urine ovulation tests to determine exact date of ovulation for precise cycle phase determination. The public health significance of this project lies in its potential to identify novel proximal, time-varying biobehavioral predictors of acute suicide risk in vulnerable adolescents to improve short-term prediction and prevention of suicidal behavior and enhance behavioral and pharmacological treatments for suicidal adolescents.